BILLINGS, Mont. - The Yellowstone cutthroat trout, an iconic fish of the West that has suffered declines in the past 200 years, will not get protection under the Endangered Species Act.

The U.S. Fish and Wildlife Service announced Tuesday it denied a petition to classify the fish as threatened or endangered.

Despite shrinking habitat and a number of recognized threats - including whirling disease, non-native predators and hybridization with other trout - there's not evidence to indicate that Yellowstone cutthroat will disappear in the next 20 to 30 years, the agency said.

"We find that the magnitude and imminence of those threats do not compromise the continued existence" of the Yellowstone cutthroat in the "foreseeable future," a Federal Register notice says.

The cutthroat, named for the reddish slash on its lower jaw, is doing fine in some places and struggling elsewhere, but it's difficult to discern an upward or downward trend in the overall population in recent years, the agency said.

A federal judge in Colorado in December 2004 ordered the agency to take a comprehensive, year-long look at the Yellowstone cutthroat after several environmental groups sued over a decision in 2001 not to list the species.

Those groups said they were disappointed by the results of the government's review but said it fits with the Bush administration's reluctance to add to the endangered species list.

The fish, they said, has been eliminated from more than 90 percent of its historic range and faces threats from mining, grazing, timber development, disease, competition from non-native species and drought.

"The Fish and Wildlife Service's finding utterly failed to consider the magnitude of the threat facing Yellowstone cutthroat trout," said Noah Greenwald of the Center for Biological Diversity. "The Yellowstone cutthroat trout is beset by a multitude of factors - and requires immediate protection under the Endangered Species Act."

The fish, one of 14 trout subspecies, once swam in long stretches of rivers and streams in Montana, Wyoming, Idaho and smaller portions of Utah and Nevada.

Today, the cutthroat occupies about 6,300 miles of streams in those states, but the population has dropped by 50 percent or more in the last 200 years, according to the Fish and Wildlife Service.

The cutthroat has been hit hard recently in Yellowstone National Park.

The population centered at Yellowstone Lake, the species' largest refuge, is a fraction of the millions that it once numbered, according to fisheries officials in the park.

Non-native lake trout have put a dent in the cutthroat population. Whirling disease and the drought have also played a role in the decline.

At two of the spawning streams, the number of fish counted by biologists has dropped more than 90 percent since 1999.

Those declines are a concern because more than 40 other species, from bald eagles to grizzly bears, feed on Yellowstone cutthroat. The fish also help support a multimillion-dollar fishing industry in the northern Rocky Mountains.

While acknowledging those problems in Yellowstone, Fish and Wildlife Service officials said their review shows that overall, the population appears stable with little change over the last decade.

The biggest single challenge to the Yellowstone cutthroat may be interbreeding with rainbow trout, which could muddy the genetic identity of the cutthroat, the agency said.

Some populations of cutthroat may not be affected, and others will. The situation is a "complex and still evolving dynamic process," the agency said.

Greenwald, with the Center for Biological Diversity, said his group plans on appealing the decision not to list the Yellowstone cutthroat.